U.S. patent application number 14/434065 was filed with the patent office on 2015-09-17 for optical sensor, and electronic apparatus.
This patent application is currently assigned to SHARP KABUSHIKI KAISHA. The applicant listed for this patent is SHARP KABUSHIKI KAISHA. Invention is credited to Norikazu Okada, Kohei Yasukawa.
Application Number | 20150263211 14/434065 |
Document ID | / |
Family ID | 50477277 |
Filed Date | 2015-09-17 |
United States Patent
Application |
20150263211 |
Kind Code |
A1 |
Okada; Norikazu ; et
al. |
September 17, 2015 |
OPTICAL SENSOR, AND ELECTRONIC APPARATUS
Abstract
A light receiving sensor (1) includes: a photodiode (PD) which
generates a photocurrent (Ipd) upon receipt of light; a transistor
(Tr11) through which the photocurrent (Ipd) flows; a transistor
(Tr12) which forms, together with the transistor (Tr11), a first
current mirror circuit (CM1); a transistor (Tr9) whose channel type
is different from that of the transistor (Tr11), and a resistor
(R10) which converts, to a voltage, a current flowing through the
transistors (Tr11 and Tr12). The first current mirror circuit (CM1)
amplifies the photocurrent (Ipd), the transistor (Tr11) has a
source connected with a gate of a MOS transistor (Tr9), and the MOS
transistor (Tr9) has a threshold voltage that is set to be equal to
or above a threshold voltage of the transistor (Tr11). This
decreases a capacity of the photodiode (PD) and therefore allows
the light receiving sensor (1) to operate at a high speed while the
photodiode (PD) is biased.
Inventors: |
Okada; Norikazu; (Osaka-shi,
JP) ; Yasukawa; Kohei; (Osaka-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SHARP KABUSHIKI KAISHA |
Osaka |
|
JP |
|
|
Assignee: |
SHARP KABUSHIKI KAISHA
Osaka-shi, Osaka
JP
|
Family ID: |
50477277 |
Appl. No.: |
14/434065 |
Filed: |
September 25, 2013 |
PCT Filed: |
September 25, 2013 |
PCT NO: |
PCT/JP2013/075903 |
371 Date: |
April 7, 2015 |
Current U.S.
Class: |
250/214A |
Current CPC
Class: |
H03F 3/08 20130101; H01L
31/02019 20130101; H01L 31/101 20130101; H03F 3/082 20130101 |
International
Class: |
H01L 31/101 20060101
H01L031/101; H03F 3/08 20060101 H03F003/08 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 12, 2012 |
JP |
2012-227412 |
Claims
1. An optical sensor comprising: a photodiode which generates a
photocurrent upon receipt of light; a first MOS transistor through
which the photocurrent flows; a second MOS transistor which forms,
together with the first MOS transistor, a current mirror circuit; a
third MOS transistor whose channel type is different from that of
the first MOS transistor; and a current-voltage conversion element
which converts, to a voltage, a current flowing through the second
MOS transistor, the current mirror circuit amplifying the
photocurrent, the first MOS transistor having a source connected
with a gate of the third MOS transistor, and the third MOS
transistor having a threshold voltage that is set to be equal to or
above a threshold voltage of the first MOS transistor.
2. The optical sensor according to claim 1, wherein a back gate of
the first MOS transistor is connected with a gate of the first MOS
transistor.
3. The optical sensor according to claim 1, further comprising: an
inverter which turns on or off the third MOS transistor by
switching on or off in accordance with a voltage into which the
photocurrent amplified by the current mirror circuit is converted,
wherein the inverter is formed by two inverter MOS transistors, one
of which has a channel type that is identical to that of the first
MOS transistor and has a back gate and a gate connected with each
other.
4. The optical sensor according to claim 3, wherein the inverter
MOS transistors are P-type MOS transistors, and the optical sensor
further includes a diode-connected N-type MOS transistor which is
an auxiliary MOS transistor connected in series with the inverter
MOS transistor.
5. The optical sensor according to claim 1, wherein the first MOS
transistor is a P-type MOS transistor, and both a drain and a gate
of the first MOS transistor are connected with a cathode of the
photodiode.
6. The optical sensor according to claim 1, wherein the first MOS
transistor is an N-type MOS transistor, and both a drain and a gate
of the first MOS transistor are connected with an anode of the
photodiode.
7. An electronic device comprising an optical sensor of claim 1.
Description
TECHNICAL FIELD
[0001] The present invention relates to an optical sensor suitable
for, for example, a photo interrupter which is used for a detection
of an object and a detection of an operating speed of an
object.
BACKGROUND ART
[0002] An electric appliance such as a digital camera and an ink
jet printer includes an operation component which is driven by a
motor. An optical sensor such as a photo interrupter is used to
detect, for example, an operating speed of the operation
component.
[0003] A certain optical sensor, among such optical sensors, has a
light receiving unit including a phototransistor and obtains a
detection signal by converting, to a voltage, a photocurrent
flowing through a phototransistor by use of an external resistance.
In this manner, a dependency of the detection signal on the
photocurrent leads to a dependency of a sensor response on the
photocurrent.
[0004] Thus, an improvement in responsiveness of the sensor is
limited. In summary, the optical sensor using a photo interrupter
is not suitable for an apparatus, such as a digital camera, for
which a high-speed operation is required. Under such circumstance,
there is a demand for an optical sensor operable at a high speed
without depending on a photocurrent.
[0005] For example, Patent Literature 1 discloses an optical sensor
(a light-receiving device) configured such that an end of a
photodiode and an end of a load are fixed to GND electric
potential. Such light-receiving device allows a prevention of a
fluctuation in a bias voltage of a photodiode.
[0006] Further, Patent Literature 2 discloses an optical sensor
configured to bias a photodiode with a gate-source voltage of an
nchMOS transistor. Even this type of optical sensor can prevent the
fluctuation in the bias voltage of the photodiode.
[0007] Both of the optical sensors disclosed in Patent Literatures
1 and 2 can prevent an electric potential difference .DELTA.V
between a cathode and an anode of the photodiode by applying the
bias voltage to the photodiode. Accordingly, it is possible to
decrease an electric charge (.DELTA.Q=C.times..DELTA.V) of a
capacitance of a photodiode, which capacitance is generated when
the photocurrent flowing through the photodiode is converted into a
voltage by the resistor. As a result, the optical sensors can
operate at a high speed.
[0008] This kind of technique can be also applied to the
phototransistor so as to allow the high-speed operation of the
optical sensor including the phototransistor.
CITATION LIST
Patent Literature
[0009] Patent Literature 1
[0010] Japanese Published Unexamined Patent Application [Japanese
Patent Application Publication No. 2001-53331 (Publication date:
Feb. 23, 2001)]
[0011] Patent Literature 2
[0012] Japanese Published Unexamined Patent Application [Japanese
Patent Application Publication No. 2012-89738 (Publication date:
May 10, 2012)]
[0013] Patent Literature 3
[0014] Japanese Patent Publication [Japanese Patent No. 3174852
(Publication date: Jun. 11, 2001)]
SUMMARY OF INVENTION
Technical Problem
[0015] FIG. 9 is a graph showing a change in capacitance of a
photodiode with respect to a voltage across an anode and a cathode
of the photodiode. FIG. 9 indicates that applying a bias voltage to
the photodiode as in Patent Literatures 1 and 2 reduces the
electric potential difference .DELTA.V, but leads to a fluctuation
of a capacitance value by the bias voltage.
[0016] Patent Literature 1 discloses a configuration in which the
cathode of the photodiode is connected with the GND. In such
configuration, a forward bias is applied to the photodiode. In a
range in which the forward bias is applied, capacitance value of
the photodiode increases, and an operating speed of the photodiode
decreases (see FIG. 9).
[0017] Note that as for a photodiode, a forward current produced by
a pn junction diode predominantly flows through the photodiode in a
region to which a forward bias of 0.7 V or above is applied. In
that region, the photodiode cannot fulfill its original
function.
[0018] An optical sensor 401 illustrated in FIG. 10 (the optical
sensor disclosed in Patent Literature 2) is configured such that a
photodiode PD1 is provided between a gate and a source of an NchMOS
transistor Tr401, and that the anode of the photodiode PD1 is
connected with the GND. Thus, the photodiode PD1 is reverse-biased.
This causes a decrease in capacitance value of the photodiode PD1.
However, in a case where a photoelectric conversion is made by
using the reverse-biased photodiode PD1 with use of a threshold
voltage of the NchMOS transistor Tr401, it is difficult to improve
an SN ratio of a detection signal if the photocurrent is small. An
impossibility to sufficiently secure the SN ratio of the detection
signal causes a disadvantage in which an operation margin of the
optical sensor 401 is decreased.
[0019] The present invention is made in view of the above problem,
and an object of the present invention is to provide an optical
sensor which can sufficiently secure the SN ratio of the detection
signal even if a small amount of light is received and can also
make a response at a high speed while a photodiode is biased.
Solution to Problem
[0020] In order to solve the above problem, an optical sensor of
one aspect of the present invention, which optical sensor
comprises: a photodiode which generates a photocurrent upon receipt
of light; a first MOS transistor through which the photocurrent
flows; a second MOS transistor which forms, together with the first
MOS transistor, a current mirror circuit; a third MOS transistor
whose channel type is different from that of the first MOS
transistor; and a current-voltage conversion element which
converts, to a voltage, a current flowing through the second MOS
transistor, the current mirror circuit amplifying the photocurrent,
the first MOS transistor having a source connected with a gate of
the third MOS transistor, and the third MOS transistor having a
threshold voltage that is set to be equal to or above a threshold
voltage of the first MOS transistor.
Advantageous Effects of Invention
[0021] According to one aspect of the present invention, a decrease
in a capacitance value of a photodiode allows the optical sensor to
operate at a high speed while the photodiode is biased. Further, it
is possible to secure a sufficient magnitude of the SN ratio of the
detection signal with respect to a small photocurrent because a
current larger than the photocurrent flowing through the resistor
cause a sufficient magnitude of the current to be secured even if
the photocurrent is small. This yields an effect of realizing a
high-speed and high-accuracy optical sensor.
BRIEF DESCRIPTION OF DRAWINGS
[0022] FIG. 1 is a circuit diagram illustrating a configuration of
a light receiving sensor of Embodiments 1 and 3 of the present
invention.
[0023] FIG. 2 is a circuit diagram illustrating a configuration of
a light receiving sensor of Embodiments 2 and 3 of the present
invention.
[0024] (a) of FIG. 3 is a sectional view illustrating a
configuration of a PchMOS transistor whose gate electrode and back
gate electrode are connected. (b) of FIG. 3 is a sectional view
illustrating a configuration of an NchMOS transistor whose gate
electrode and back gate electrode are connected.
[0025] FIG. 4 is a graph showing characteristics of currents
relative to a gate voltage for a back gate-gate connection and a
back gate-source connection of a MOS transistor which is connected
with a photodiode in the light receiving sensor illustrated in FIG.
1.
[0026] FIG. 5 is a circuit diagram illustrating a configuration of
a light receiving sensor of Embodiment 4 of the present
invention.
[0027] FIG. 6 is a circuit diagram illustrating a configuration of
a light receiving sensor of Embodiment 5 of the present
invention.
[0028] (a) of FIG. 7 illustrates a simulation result of a response
property relative to an NchMOS transistor when a detection signal
of the light receiving sensor illustrated in FIG. 5 is changed from
a high level to a low level. (b) of FIG. 7 illustrates a simulation
result of a response property relative to an NchMOS transistor when
a detection signal of the light receiving sensor illustrated in
FIG. 6 is changed from a high level to a low level.
[0029] FIG. 8 is a front view illustrating an inner structure of a
copying machine of Embodiment 6 of the present invention.
[0030] FIG. 9 is a graph showing a property of a capacitance
relative to a voltage across an anode and a cathode of a
photodiode.
[0031] FIG. 10 is a circuit diagram illustrating a configuration of
a conventional optical sensor.
DESCRIPTION OF EMBODIMENTS
Embodiment 1
[0032] The following description discusses Embodiment 1 of the
present invention with reference to FIG. 1.
[Configuration of Light Receiving Sensor]
[0033] FIG. 1 is a circuit diagram illustrating a configuration of
a light receiving sensor 1 of the present embodiment.
[0034] As illustrated in FIG. 1, the light receiving sensor 1 (an
optical sensor) includes a photodiode PD, a first current mirror
circuit CM1, a transistor Tr9, a resistor R10 (a current-voltage
conversion element), and a constant-current source 3.
[0035] The photodiode PD is a photoelectric conversion element
which, upon receipt of incoming light, passes a photocurrent Ipd.
An anode of the photodiode PD is connected with a ground line to
which a GND electric potential is given. Further, as described
below, a cathode of the photodiode PD is connected with the first
current mirror circuit CM1.
[0036] The first current mirror circuit CM1 includes a pair of
PchMOS transistors: transistors Tr11 and Tr12 (P-type MOS
transistors). A drain of the transistor Tr11 (a first MOS
transistor) on an input side is connected with each a cathode of
the photodiode PD and a gate of the transistor Tr11. Further, a
source of the transistor Tr11 is connected with an output terminal
via a resistor R10. A drain of the transistor Tr12 on an output
side is connected with a ground line, and a source of the
transistor Tr12 is connected with the source of the transistor
Tr11. Still further, a gate of the transistor Tr12 is connected
with the gate of the transistor Tr11.
[0037] The first current mirror circuit CM1 is configured such that
a dimension ratio of the transistors Tr11 and Tr12 is 1:n (n>1).
Thus, the first current mirror circuit CM1 serves as a current
amplifier which amplifies the photocurrent Ipd by n times. Upper
limit of the value of n is around several tens.
[0038] The transistor Tr9 (a third MOS transistor) is an NchMOS
transistor. A source of the transistor Tr9 is connected with a
ground line and a drain of the transistor Tr9 is connected with the
output terminal. Further, a gate of the transistor Tr9 is connected
with each source of the transistors Tr11 and Tr12. To the output
terminal, an output voltage Vout as the detection signal is
outputted.
[0039] A constant-current source 3 is connected between the output
terminal and a power-supply line to which a constant voltage Vdd is
applied and is provided to flow constant currents through a
resistor R10 and a transistor Tr9.
[Operation of Light Receiving Sensor]
[0040] The light receiving sensor 1 as configured above operates as
follows.
[0041] Upon receipt of light, the photodiode PD generates the
photocurrent Ipd. Accordingly, the transistors Tr11 and Tr12 in the
first current mirror circuit CM1 are turned on. Further, an
on-operation of the transistor Tr9 is continued because the
transistor Tr9 is biased by the constant-current source 3
regardless of presence or absence of a light input.
[0042] Still further, the first current mirror circuit CM1
amplifies the photocurrent Ipd by n times to flow, through the
transistor Tr12, an amplified photocurrent lamp which is n times
larger than the photocurrent Ipd. This causes the first current
mirror circuit CM1 to pass, through the resistor R10, a current
that is (n+1) times larger than the photocurrent Ipd. This current
is converted to a voltage by the resistor R10. This voltage is the
output voltage Vout outputted as a high level detection signal from
the output terminal, and the output voltage Vout is represented by
the following expression:
Vout=(n+1).times.Ipd.times.R10+Vth9,
where Ipd is a current value of the photocurrent Ipd, R10 is a
resistance value of the resistor R10, and Vth9 is a threshold
voltage of the transistor Tr9.
[0043] Further, a value of a bias voltage of the photodiode PD
corresponds to a value obtained by subtracting a threshold voltage
of the transistor Tr11 from a threshold voltage of the transistor
Tr9. Thus, it is preferable that the threshold voltage of the
transistor Tr9 is equal to or above the threshold voltage of the
transistor Tr11. Accordingly, it is possible to reduce a
capacitance of the photodiode PD.
[0044] On the other hand, the photodiode PD not receiving light
does not generate the photocurrent Ipd. Thus, both of the
transistors Tr11 and Tr12 in the first current mirror circuit CM1
are off. Further, a voltage from the power-supply line is applied
to the gate of the transistor Tr9 via the resistor R10.
Accordingly, an electric potential of the output terminal is
decreased to the threshold voltage Vth9 of the transistor Tr9.
Thus, a decrease in the electric potential of the output terminal
causes a low level detection signal to appear at the output
terminal.
[Effects Obtained by Light Receiving Sensor]
[0045] As described earlier, the light receiving sensor 1 of the
present embodiment includes the first current mirror circuit CM1
and the transistor Tr9. The first current mirror circuit CM1
amplifies the photocurrent Ipd by n times. The transistor Tr9 has a
drain and a source being connected with the output terminal and the
ground line, respectively. Further, each source of the transistors
Tr11 and Tr12 in the first current mirror circuit CM1 is connected
with the gate of the transistor Tr9. Still further, the threshold
voltage of the transistor Tr9 is set so as to be equal to or above
the threshold voltage of the transistor Tr11.
[0046] This decreases a capacitance value of the photodiode PD
causes an operating speed of the photodiode PD to be improved.
Further, a current flowing through the resistor R10 is larger than
the photocurrent Ipd so that the light receiving sensor 1 can
largely reduce a dependency of the detection signal on the
photocurrent Ipd. Accordingly, the light receiving sensor 1 can
operate at a high speed. Therefore, the light receiving sensor 1
can be suitably used for an apparatus which has to detect, at a
high speed, an object to be inspected.
[0047] Further, a current larger than the photocurrent Ipd flows
through the resistor R10. Thus, it is possible to ensure that the
current flowing through the resistor R10 has a sufficiently large
current even if a magnitude of the photocurrent Ipd is very small.
Accordingly, it is possible to secure a sufficiently large SN ratio
of the detection signal (the output voltage Vout) with respect to
the very small photocurrent Ipd.
Embodiment 2
[0048] The following description discusses Embodiment 2 of the
present invention with reference to FIG. 2.
[0049] Note that in Embodiment 2, identical reference signs are
given to components having the same functions as components in
Embodiment 1, and descriptions of the components are omitted.
[Configuration of Light Receiving Sensor]
[0050] FIG. 2 is a circuit diagram illustrating a configuration of
a light receiving sensor 2 of the present embodiment.
[0051] As illustrated in FIG. 2, the light receiving sensor 2 (an
optical sensor) includes a photodiode PD, a first current mirror
circuit CM11, a transistor Tr10, a resistor R10, and a
constant-current source 3.
[0052] The light receiving sensor 2 is configured such that a
cathode of the photodiode PD is connected with the power-supply
line.
[0053] The first current mirror circuit CM11 includes a pair of
NchMOS transistors: the transistors Tr111 and Tr112 (N-type MOS
transistors). A drain of the transistor Tr111 on an input side (a
first MOS transistor) is connected with each an anode of the
photodiode PD and a gate of the transistor Tr111. Further, a source
of the transistor Tr111 is connected with an output terminal via
the resistor R10. A drain of the transistor Tr112 on an output side
is connected with the power-supply line and a source of the
transistor Tr112 is connected with the source of the transistor
Tr111. Still further, a gate of the transistor Tr112 is connected
with the gate of the transistor Tr111.
[0054] The first current mirror circuit CM11 is configured such
that a dimension ratio of the transistor Tr111 and the transistor
Tr112 is 1:n(n>1). Thus, the first current mirror circuit CM11
serves as a current amplifier which amplifies the photocurrent Ipd
by n times.
[0055] The transistor Tr10 (a third MOS transistor) is a PchMOS
transistor whose channel type is different from channel types of
the transistors Tr111 and Tr112. A source of the transistor Tr10 is
connected with the power-supply line and a drain of the transistor
Tr10 is connected with the output terminal. Further, a gate of the
transistor Tr10 is connected with each source of the transistors
Tr111 and Tr112. To the output terminal, the output voltage Vout as
the detection signal is outputted.
[0056] The constant-current source 3 is connected between the
power-supply line and the output terminal and is provided to flow
constant currents through the resistor R10 and the transistor
Tr10.
[Operation of Light Receiving Sensor]
[0057] The light receiving sensor 2 as configured above operates as
follows.
[0058] Upon receipt of light, the photodiode PD generates the
photocurrent Ipd. Accordingly, the transistors Tr111 and Tr112 in
the first current mirror circuit CM11 are turned on.
[0059] Further, the first current mirror circuit CM11 amplifies the
photocurrent Ipd by n times. This causes the first current mirror
circuit CM11 to pass, through the resistor R10, a current that is
(n+1) times larger than the photocurrent Ipd. The current is
converted to a voltage by the resistor R10. The voltage obtained by
the conversion is added to the threshold voltage of the transistor
Tr10, and a resultant voltage is then subtracted from the constant
voltage Vdd so as to obtain an output voltage Vout as represented
by the above expression. The output voltage Vout is outputted as a
low level detection signal from the output terminal.
[0060] Still further, a value obtained by subtracting the threshold
voltage of the transistor Tr111 from the threshold voltage of the
transistor Tr10 is 0 V or above. Accordingly, a bias voltage of a
reverse bias is applied to the photodiode PD.
[0061] On the other hand, the photodiode PD not receiving inputted
light does not generate the photocurrent Ipd. Thus, the first
current mirror circuit CM11 is configured such that both
transistors Tr111 and Tr112 are off. A value of an electric
potential of the output terminal corresponds to a value obtained by
subtracting the threshold voltage of the transistor Tr10 from the
constant voltage Vdd so that a high level detection signal appears
at the output terminal.
[Effects Obtained by Light Receiving Sensor]
[0062] As described earlier, the light receiving sensor 2 of the
present embodiment includes the first current mirror circuit CM11
and the transistor Tr10. The first current mirror circuit CM11
amplifies the photocurrent Ipd by n times. The transistor Tr10 has
a drain and a source being connected with the output terminal and
the ground line, respectively. Further, each source of the
transistors Tr111 and Tr112 in the first current mirror circuit
CM11 is connected with the gate of the transistor Tr10. Still
further, the threshold voltage of the transistor Tr10 is set so as
to be equal to or above the threshold voltage of the transistor
Tr111.
[0063] Accordingly, as similar to the light receiving sensor 1, the
light receiving sensor 2 can largely reduce a dependency of the
detection signal on the photocurrent Ipd because the current
flowing through the resistor R10 is larger than the photocurrent
Ipd. Accordingly, the light receiving sensor 2 can operate at a
high speed. Therefore, the light receiving sensor 2 can be suitably
used for an apparatus which has to detect, at a high speed, an
object to be inspected.
[0064] Further, as in the light receiving sensor 1, a current
larger than the photocurrent Ipd flows through the resistor R10.
Thus, it is possible to ensure that the current flowing through the
resistor R10 has a sufficiently large current even if the
photocurrent Ipd is very small. Accordingly, it is possible to
secure a sufficiently large SN ratio of the detection signal (the
output voltage Vout) with respect to the very small photocurrent
Ipd.
Embodiment 3
[0065] The following description discusses Embodiment 3 of the
present invention with reference to FIGS. 1 to 4.
[0066] Note that in Embodiment 3, identical reference signs are
given to components having the same function of components in
Embodiments 1 and 2, and descriptions of the components are
omitted.
[Configuration of Light Receiving Sensor]
[0067] (a) of FIG. 3 is a sectional view of a structure of a PchMOS
transistor in which a gate electrode and a back gate electrode are
connected with each other. (b) of FIG. 3 is a sectional view of an
NchMOS transistor in which a gate electrode and a back gate
electrode are connected with each other.
<Light Receiving Sensor 1>
[0068] The light receiving sensor 1 is configured such that a gate
and a back gate of the transistor Tr11 are connected with each
other.
[0069] As illustrated in (a) of FIG. 3, normally in a case where a
PchMOS transistor 101 (the P-type MOS transistor) is to be provided
on a p substrate 103, the PchMOS transistor 101 is provided in an n
well 104 which is provided in the p substrate 103. In the n well
104, a p-type diffusion layer 105 forming a source and a p-type
diffusion layer 106 forming a drain are provided. Further, in the n
well 104, a channel is provided between the p-type diffusion layers
105 and 106. On the n well 104, a gate 107 is provided on a region
thereof corresponding to the channel.
[0070] The source electrode 111 and the drain electrode 112 are
connected with the p-type diffusion layer 105 and the p-type
diffusion layer 106, respectively. Further, the gate electrode 113
is connected with the gate 107.
[0071] A MOS transistor is usually configured such that a substrate
serves as a gate. Thus, the gate is treated as a back gate. The
PchMOS transistor 101 is configured such that the n well 104 serves
as a substrate. Thus, the n well 104 is treated as a back gate.
Accordingly, back gate electrodes 114 and 115 are each connected
with the n well 104. Further, a gate electrode 113 is connected
with each of the back gate electrodes 114 and 115.
[0072] A connection structure of the electrodes in the PchMOS
transistor 101 as described above is applied to the transistor
Tr11.
<Light Receiving Sensor 2>
[0073] The light receiving sensor 2 is configured such that a gate
and a back gate of the transistor Tr111 are connected with each
other.
[0074] As illustrated in (b) of FIG. 3, an NchMOS transistor 102
(an N-type MOS transistor) is configured such that the n well 104
is provided on the p substrate 103, and a p well 110 is formed in
the n well 104. On the p well 110, an n-type diffusion layer 108
forming a source and an n-type diffusion layer 109 forming a drain
are provided. Further, in the p substrate 103, a channel is
provided between the n-type diffusion layers 108 and 109. On the p
substrate, a gate 107 is provided on a region thereof corresponding
to the channel.
[0075] The source electrode 111 and the drain electrode 112 are
connected with the n-type diffusion layer 108 and the n-type
diffusion layer 109, respectively. Further, the gate electrode 113
is connected with the gate 107.
[0076] The NchMOS transistor 102 is configured such that the p-well
110 serves as a back gate. Thus, back gate electrodes 114 and 115
are each connected with the p well 110. Further, the gate electrode
113 is connected with each of the back gate electrodes 114 and
115.
[0077] A connection structure of the electrodes in the NchMOS
transistor 102 as described above is applied to the transistor
Tr111.
[Effects Obtained by Light Receiving Sensor]
[0078] FIG. 4 is a graph showing characteristics of currents
relative to a gate voltage for a back gate-gate connection (a
dynamic threshold MOS transistor) and relative to a gate voltage
for a back gate-source connection in the transistor Tr11. Patent
Literature 3 discloses the dynamic threshold MOS transistor.
[0079] A MOS transistor usually generates a leakage current between
a drain and a source even at a gate-source voltage of 0 V when a
diffusive concentration is changed to decrease a threshold voltage.
At the occurrence of this kind of phenomenon, the photocurrent Ipd
does not decrease even when the light receiving sensors 1 and 2 do
not receive light. This may cause the light receiving sensors 1 and
2 to make a false detection.
[0080] In view of this, the light receiving sensor 1 is configured
such that a back gate and a gate of the transistor Tr11, which is
the PchMOS transistor, are connected. Further, the light receiving
sensor 2 is configured such that a back gate and a gate of the
transistor Tr111, which is the NchMOS transistor, are connected.
Accordingly, it is possible to decrease threshold voltages of the
transistors Tr11 and Tr111 without changing a diffusive
concentration. This leads to a decrease of a leakage current, and
thus, a reverse bias voltage of the photodiode PD can be further
increased. Consequently, it is possible to improve response speeds
of the light receiving sensors 1 and 2. In summary, the back gate
and gate of each of the transistors Tr11 and Tr111 are beneficially
connected.
[0081] FIG. 4 shows the result of simulations of current values of
(n+1).times.Ipd with respect to a gate-source voltage of the
transistor Tr11 in the transistor Tr11 in which the back gate is
connected with the gate and in the transistor Tr11 in which the
back gate is connected with the source.
[0082] As is evident from FIG. 4, the transistor Tr11 in which the
back gate is connected with the gate decrease the threshold voltage
by around 60 mV with a current value of 50 nA as compared with the
transistor Tr11 in which the back gate is connected with the
source.
Embodiment 4
[0083] The following description discusses Embodiment 4 of the
present invention with reference to FIG. 5.
[0084] Note that in Embodiment 4, identical reference signs are
given to components having the same functions as components in
Embodiments 1 through 3, and descriptions of the components are
omitted.
[Configuration of Light Receiving Sensor]
[0085] FIG. 5 is a circuit diagram illustrating a configuration of
a light receiving sensor 10 of the present embodiment.
[0086] As illustrated in FIG. 5, the light receiving sensor 10 (an
optical sensor) includes a light receiving element 15 and an
external resistor RL.
[0087] The light receiving element 15 includes two terminals T1 and
T2, a resistor R11, a detection signal generation unit 21, and a
zero bias circuit 22. The light receiving element 15 is a
two-terminal optical detection circuit which, upon receipt of
light, fluctuates a circuit current and, in turn, fluctuates an
electric potential of the terminal T1 relative to a fixed potential
of the terminal T2 so as to output a detection signal.
(1) Configuration of Terminal
[0088] The terminal T1 (a first terminal) serves as both an output
terminal from which a detection signal is outputted and a supply
terminal to which a power source voltage Vcc is applied. The
terminal T1 is connected with a power-supply line via the external
resistor RL. The terminal T2 (a second terminal) is a grounding
terminal which is connected with a ground line and to which a
grounding potential (a fixed potential) is given.
[0089] Note that the terminal T1 can be a terminal to which a fixed
potential is given and the terminal T2 can be a terminal with
fluctuating electric potentials.
[0090] The light receiving element 15 is a circuit which, upon
receipt of light directly from a light emitting element (not shown)
or upon receipt of light reflected by an object, converts the light
to an electric signal (a detection signal) and then outputs the
electric signal.
(2) Configuration of Detection Signal Generation Unit
[0091] The detection signal generation unit 21 includes a
photodiode PD, resistors R1 and R2, transistors Tr1 to Tr5 (MOS
transistors), and a first current mirror circuit CM1.
[0092] The photodiode PD has (i) an anode connected with the
terminal T2 and (ii) a cathode connected with a source of the
transistor Tr15 in the zero bias circuit 22.
[0093] Note that the detection signal generation unit 21 includes
the photodiode PD as a photoelectric conversion element, but may
include a phototransistor instead of the photodiode PD.
[0094] In the first current mirror circuit CM1, a drain of the
transistor Tr11 on an input side is connected with a drain of the
transistor Tr15 in a zero bias circuit 22 described later and with
a gate of the transistor Tr11. Further, a source of the transistor
Tr11 is connected with the terminal T1. A drain of the transistor
Tr12 on an output side is connected with one end of the resistor R1
and with a gate of the transistor Tr1. A source of the transistor
Tr12 is connected with the terminal T1.
[0095] Another end of the resistor R1 and a source of the
transistor Tr1 are connected with the terminal T2. A drain of the
transistor Tr1 is connected with a drain of the transistor Tr2 and
with a gate of the transistor Tr4. A source of the transistor Tr2
is connected with a drain of the transistor Tr3, and a gate of the
transistor Tr2 is connected with the gate of the transistor Tr1.
The transistors Tr1 and Tr2 form an inverter by being connected in
a manner as described above.
[0096] Further, the transistor Tr2 (an inverter MOS transistor) has
a channel type which is the same as that of the transistor Tr1.
Like the transistor Tr11, the transistor Tr2 is configured such
that a back gate and a gate are connected with each other.
[0097] A source of the transistor Tr3 is connected with the
terminal T1, and a gate of the transistor Tr3 is connected with the
drain of the transistor Tr3. Accordingly, the transistor Tr3
functions as a diode.
[0098] A source of the transistor Tr4 is connected with the
terminal T2, and a drain of the transistor Tr4 is connected with a
source of the transistor Tr5. A drain of the transistor Tr5 is
connected with the terminal T1 and a gate of the transistor Tr5 is
connected with a gate of the transistor Tr4 and with one end of the
resistor R2. Another end of the resistor R2 is connected with the
terminal T1.
(3) Configuration of Zero Bias Circuit
[0099] The zero bias circuit 22 includes the transistors Tr13 and
Tr15. As described earlier, the transistor Tr15 is connected with
the photodiode PD and with the transistor Tr11 in the first current
mirror circuit CM1. Further, a drain of the transistor Tr13 is
connected with the terminal T1 via a resistor R11. Still further, a
gate of the transistor Tr15 is connected with a gate of the
transistor Tr13. The transistors Tr13 and Tr15 form a current
mirror circuit with their gates connected with each other. Further,
the transistors Tr13 and Tr15 form a grounded gate circuit.
[Operation of Light Receiving Sensor]
(1) Basic Operation
[0100] The photodiode PD, upon receipt of light, flows the
photocurrent Ipd. The photocurrent Ipd is amplified by the first
current mirror circuit CM1, flows through the resistor R1, and is
then converted to a voltage by the resistor R1.
[0101] Accordingly, potentials of gates of the transistors Tr1 and
Tr2 fluctuate. Then, a resistance value of the resistor R1 is so
set that the gate potential exceeds a threshold voltage of the
inverter when a value of the photocurrent Ipd is equal to or above
a given value.
[0102] In a case where a current flowing through the resistor R1
upon receipt of light is converted into a voltage, the light which
exceeds a threshold voltage of the inverter formed by the
transistors Tr1 and Tr2 causes the transistor Tr2 to be turned off
and causes the transistor Tr1 to be turned on. Accordingly, the
transistor Tr4 is turned off to stop a flow of current (a terminal
current) of the transistor Tr4. This increases a voltage (an
electric potential difference) between the terminals T1 and T2 (two
terminals).
[0103] On the other hand, when a decrease in a quantity of incoming
light leads to a decrease of the photocurrent Ipd, a current
amplified with the first current mirror circuit CM1 is decreased.
This decreases a voltage across the terminals of the resistor R1.
In a case where gate-source voltages of the transistors Tr1 and Tr2
are decreased to a threshold voltage of the inverter, the
transistor Tr2 is turned on while the transistor Tr1 is on.
Accordingly, the transistor Tr4 is turned on to decrease a voltage
between the two terminals.
[0104] In this manner, a voltage between the two terminals is
increased or decreased depending on whether the transistor Tr4 is
off or on. Thus, a detection signal which appears between the two
terminals, when incoming light is present is a high level voltage,
and a detection signal which appears when incoming light is not
present is a low level voltage. Specifically, when incoming light
is present, a very small voltage drop is caused between two
terminals by the photocurrent Ipd and a drive current of the
transistor Tr1. On the other hand, when incoming light is not
present, an output current of the light receiving element 15 is
determined by a drive current of the transistor Tr4. Due to a
voltage drop caused by the output current, an output voltage of the
light receiving element 15 goes a low level. Detection performance
improves with increase in difference between a high level output
voltage and a low level output voltage of the light receiving
element 15. Thus, an increase in drive current of the transistor
Tr4 reduces an influence of the photocurrent Ipd.
[0105] Further, when the transistor Tr4 is on/off, the transistor
Tr1 necessarily performs an on-operation, or both of the
transistors Tr1 and Tr2 necessarily performs an on-operation.
Accordingly, the transistor Tr4 can operate at a higher speed.
Thus, a response speed of the light receiving sensor 10 can be
improved.
[0106] Incidentally, the light receiving element 15 is configured
such that a source voltage of the transistor Tr2 has to be
decreased because the transistor Tr2 has to be switched depending
on a voltage across the terminals of the resistor R1. In other
words, it is necessary to prevent switching of the transistor Tr2
by depending on a source-drain voltage of the first current mirror
circuit CM1. Thus, the transistor Tr3 which serves as a diode is
arranged in series with the transistor Tr2.
[0107] Further, with this arrangement, operating points of the
inverter at the change in output voltage of the light receiving
element 15 from a high level to a low level and from at the change
in output voltage from a low level to a high level can vary
according to a drop and rise of a voltage between the two
terminals. Thus, it is possible to obtain a hysteresis
property.
(2) Prevention of Decrease in Response Speed
[0108] As described earlier, the transistors Tr1 and Tr2 form the
inverter. This allows the transistor Tr4 to be switched at a high
speed. However, in a case where an electric potential difference
between two terminals is small, an electric potential difference
between a gate and a drain of the transistor Tr2 gradually becomes
small after a start of switching of the transistor Tr4. This
reduces the current. Accordingly, a response speed of the light
receiving element 15 gradually decreases.
[0109] Then, in order to avoid such inconvenience, the resistor R2
is provided in the light receiving element 15. Accordingly, a
decrease in a voltage between two terminals is assisted with the
resistor R2 so that a decrease in response speed can be
prevented.
[0110] Meanwhile, if each transistor in the detection signal
generation unit 21 in the light receiving element 15 is a MOS
transistor, an adjustment of a dose amount can change an operation
threshold level of the transistor. For example, an operation
threshold level of a current generation transistor (the transistor
Tr4 in the light receiving element 15) is set to 0.7 V or below so
as to cause an electric potential difference between two terminals.
This is because normally, a detection signal reception device in
the light receiving sensor 10 sets a threshold level to be a diode
voltage, in other words, to be 0.7 V or above.
[0111] Accordingly, an electric potential difference between two
terminals can be made larger so as to widen an operation range of
the light receiving element 15.
(3) Reduction in Leakage Current
[0112] If the light receiving element 15 uses a transistor with a
low threshold level, it is concerned that a leakage current is
caused when the transistor Tr4 is off under a high temperature.
Causing such leakage current leads to an inconvenience in which an
electric potential difference between two terminals is decreased
even though, in nature, an electric potential difference between
two terminals has to be increased.
[0113] Then, in order to avoid such inconvenience, the transistor
Tr5 to be cascade-connected with the transistor Tr4 is provided.
With this arrangement, a decrease in a drain voltage of the
transistor Tr4 allows a decrease in the leakage current when the
transistor Tr4 is off by 1/10 or more. Thus, it is possible to
largely reduce the leakage current when the transistor Tr4 is off.
Especially, the transistor Tr4 has to be formed in a large size so
that the transistor Tr4 which makes a switching operation flows a
large magnitude of current, and thus, a leakage current easily
becomes large accordingly. Therefore, it is possible to prevent a
decrease in an electric potential difference between two terminals
at the rise of an electric potential difference between two
terminals.
[0114] Meanwhile, the transistor Tr1 may operate falsely if a
fluctuation in a temperature property of a threshold level is
large. This is because the threshold level of the MOS transistor is
decreased under a high temperature. On the other hand, if, for
example, a diffused resistor is used for a current-voltage
conversion, sensitivity comes to have a large temperature property,
because a resistance value rises under a high temperature.
[0115] In view of this, the light receiving element 15 is
configured such that the resistor R1 (the bias resistor) is
composed of a resistor (e.g., polysilicon resistor) having a
negative temperature property so that such inconvenience can be
prevented. Accordingly, a temperature property of the transistor
Tr1 which is a MOS transistor and a temperature property of the
resistor R1 can be cancelled out. As a result, it is possible to
prevent a fluctuation in a temperature property of the light
receiving element 15.
(4) Operation of Zero Bias Circuit
[0116] The zero bias circuit 22 is configured such that a potential
of a source of the transistor Tr13 is a GND potential (a grounding
potential) and each gate of the transistors Tr13 and Tr15 is a gate
potential. Accordingly, a source of the transistor Tr15 is also a
GND.
[0117] Therefore, an electric potential difference between an anode
and a cathode of the photodiode PD is zero. Further, the transistor
Tr15 is configured such that a source signal itself is a drain
signal. As a result, there is no problem in a transmission of a
signal.
[Effect Obtained by Light Receiving Section]
(1) Digitization of Detection Signal
[0118] It is necessary that a value of a current flowing between
the terminals T1 and T2 of the light receiving sensor 10 is larger
than a value of the photocurrent Ipd so as to depend on a current
controlled by the transistor Tr4. For example, when the
photocurrent Ipd takes a value up to several .mu.A, the current
controlled by the transistor Tr4 takes a value up to several
mA.
[0119] Accordingly, the photocurrent Ipd is only used for a
switching control of the transistor Tr4, so that the detection
signal of the light receiving sensor 10 does not depend on the
photocurrent Ipd. Thus, the detection signal to be outputted can be
binary signals with a high level and a low level. In this manner, a
use of the light receiving sensor 10 allows generating detection
signal in digital form rather than in analog form. Thus, it is
possible to cause the light receiving sensor 10 to be operated at a
high speed.
[0120] Further, as described earlier, the digitization of the
detection signal allows the light receiving sensor 10 to have a
hysteresis property. This eliminates the need to provide a
hysteresis circuit for converting an analog detection signal to a
digital detection signal, like a conventional analog output sensor.
Thus, a use of the light receiving sensor 10 is beneficial also in
terms of a simplification of a circuit configuration.
(2) Improvement in Response Property
[0121] The light receiving sensor 10 is configured such that a
source of the transistor Tr11 in the light receiving sensor 1 of
Embodiment 1 is connected with a gate of the transistor Tr4 (the
NchMOS transistor) via the resistor R2.
[0122] A current having the same magnitude as that of the current
((n+1).times.Ipd) flowing through the transistors Tr11 and Tr12
flows through a channel under the gate of the transistor Tr4. The
current is converted to a voltage by a channel resistor of the
transistor Tr4. Accordingly, when light is not received, a voltage
between the terminal T1 and the terminal T2 is decreased to a
threshold voltage of the transistor Tr4.
[0123] Meanwhile, unlike the light receiving sensor 1, the light
receiving sensor 10 does not include a constant-current source 3,
so that a zero bias circuit 22 biases the photodiode PD. This is
because the light receiving sensor 10, as in the light receiving
sensor 1, is configured such that the photodiode PD is biased until
the transistor Tr1 is turned on, but the photodiode PD is not
biased when the transistor Tr4 is turned off.
[0124] Alternatively, the circuit of the light receiving sensor 10
may be arranged such all the MOS transistors in the light receiving
sensor 10 is replaced with MOS transistors whose channel type is
opposite to that of the MOS transistors in the light receiving
sensor 10 so that the light receiving sensor 2 of Embodiment 2 is
applied to the light receiving sensor 10.
[0125] In the light receiving sensor 10 having such an arrangement,
a speed of change of the detection signal from a high level to a
low level is determined according to an operating speed of the
transistor Tr2. In view of this, by connecting a back gate and a
gate of the transistor Tr2, a threshold voltage can be decreased as
in the transistor Tr11 in Embodiment 3. Accordingly, the transistor
Tr2 can be driven with a low voltage. Thus, an increase in
operating speed of the transistor Tr4 allows the light receiving
sensor 10 to be operated at a high speed. This is beneficial in
improving a response property of the light receiving sensor.
[0126] Further, the light receiving sensor 10 is configured such
that, upon receipt of light, the photodiode PD operates as
described earlier, so that a value of an electric potential of the
terminal T1 is almost the same as a value of the power source
voltage Vcc. However, a small voltage drop is caused by the
photocurrent Ipd. Receipt of light having an amount equal to or
more than a predetermined amount causes a rise of a voltage of the
resistor R1. This limits an operation current of the transistor
Tr12. Thus, a voltage drop is limited.
[0127] On the other hand, when an amount of light received by the
photodiode PD is decreased, the light receiving sensor 10 operates
as described earlier, so that a voltage between the terminals T1
and T2 (a voltage between terminals) is decreased to a voltage
determined by the threshold voltage of the transistor Tr4. Thus, a
value of a voltage between the terminals corresponds to a value
obtained by subtracting the threshold voltage from the power source
voltage Vcc.
[0128] In this manner, if a switching control of the transistor Tr4
is made by depending on the photocurrent, in detecting light, a
maximum electric potential difference between two terminals is
determined in accordance with a threshold voltage of the transistor
Tr4. Thus, setting the threshold voltage to be as low as 0.5 V or
below allows an electric potential difference between terminals to
be in a wide range obtained by subtracting a threshold voltage (0.5
V or below) from a power source voltage Vcc (a fixed
potential).
(3) Zero Biasing of Photodiode
[0129] The zero bias circuit 22 causes a bias voltage of the
photodiode PD to be zero. This eliminates the need for the
photodiode PD charging its own capacitance even when the
photocurrent Ipd flows in. This is desirable because a signal
response speed of the light receiving sensor 10 can be
increased.
Embodiment 5
[0130] The following description discusses Embodiment 5 of the
present invention with reference to FIG. 6.
[0131] Note that in Embodiment 5, identical reference signs are
given to components having the same functions as components in
Embodiment 4, and descriptions of the components are omitted.
[Configuration of Light Receiving Sensor]
[0132] FIG. 6 is a circuit diagram illustrating a configuration of
a light receiving sensor 11 of the present embodiment.
[0133] As illustrated in FIG. 6, the light receiving sensor 11 (the
optical sensor) includes a light receiving element 16 and an
external resistor RL.
[0134] As in the light receiving element 15 of Embodiment 4
described earlier, the light receiving element 16 includes two
terminals T1 and T2, the resistor R11, and the zero bias circuit
22. Further, the light receiving element 16 includes a detection
signal generation unit 24 instead of the detection signal
generation unit 21 in the light receiving element 15.
[0135] As in the detection signal generation unit 21, the detection
signal generation unit 24 includes the photodiode PD, the resistors
R1 and R2, the transistors Tr1, Tr2, Tr4, and Tr5 (the MOS
transistors), and the first current mirror circuit CM1. Further,
the detection signal generation unit 24 includes the transistor Tr6
instead of the transistor Tr3 in the detection signal generation
unit 21.
[0136] The transistor Tr6 (the auxiliary MOS transistor) is the
NchMOS transistor. A drain of the transistor Tr6 is connected with
a terminal T1 and with a gate of the transistor Tr6. Such diode
connection causes the transistor Tr6 to serve as a diode. Further,
a source of the transistor Tr6 is connected with a source of the
transistor Tr2.
[Effect Obtained by Light Receiving Section]
[0137] As in the light receiving sensor 10 of Embodiment 4, the
light receiving sensor 11 is also configured such that a source of
the transistor Tr11 in the light receiving sensor 1 of Embodiment 1
is connected with a gate of the transistor Tr4 (the NchMOS
transistor) via the resistors R31 and R2.
[0138] The light receiving sensor 11 having such a configuration
can decrease a threshold voltage in the same manner as in the
transistor Tr11 of Embodiment 3, by connecting a back gate of the
transistor Tr2 with a gate of the transistor Tr2. Accordingly, the
transistor Tr2 can be driven with a low voltage. As a result, the
light receiving sensor 11 can operate at a high speed so that a
response property of the light receiving sensor can be
improved.
[0139] Further, the detection signal generation unit 24 includes
the transistor Tr6 which is a diode-connected NchMOS transistor. In
other words, the light receiving sensor 11 includes the transistor
Tr6 instead of the transistor Tr3 in the light receiving sensor 10.
Accordingly, a decrease in threshold voltage of the NchMOS
transistor in the light receiving element 16 leads to a similar
decrease in threshold voltage of the transistor Tr6. Therefore, a
response characteristic of the light receiving sensor 11 can be
improved as compared to a response characteristic of the light
receiving sensor 10. Reasons thereof are described below.
[0140] (a) of FIG. 7 illustrates the result of a simulation of a
response property with respect to the NchMOS transistor when the
detection signal of the light receiving sensor 10 changes from a
high level to a low level. (b) of FIG. 7 illustrates the result of
a simulation of a response property with respect to the NchMOS
transistor when the detection signal of the light receiving sensor
11 changes from a high level to a low level. (a) and (b) of FIG. 7
illustrate a fall time TPHL in which the detection signal changes
from a high level to a low level relative to the threshold voltage
(Vth) of the transistor Tr2. As illustrated in (a) of FIG. 7, in
the light receiving sensor 10, a fall time TPHL relative to any
value of Vth at -25.degree. C. (shown in a circle), 25 C (shown in
a triangle), and 85.degree. C. (shown in a square) is distributed
in a range of 9 to 18 .mu.S. On the other hand, as illustrated in
(b) of FIG. 7, in the light receiving sensor 11, a fall time TPHL
relative to any value of Vth under -25.degree. C., 25.degree. C.,
and 85.degree. C. is distributed in a range of 5.5 to 11 .mu.S.
Especially, in a range where Vth is lower than 0.2 V, a fall time
TPHL can be shortened in the light receiving sensor 11 ((b) of FIG.
7) than in the light receiving sensor 10 ((a) of FIG. 7).
[0141] In this manner, the light receiving sensor 11 includes the
transistor Tr6 to reduce a dependency on the NchMOS transistor so
that a response delay can be prevented. Accordingly, a response
property of the light receiving sensor can be beneficially
improved.
Embodiment 6
[0142] Each light receiving sensors 1, 2, 10, and 11 of Embodiments
1 through 5 described earlier is suitably used for an electronic
device such as a digital camera, a copying machine, a printer, and
a portable device, each using a photo interrupter. Further, the
light receiving sensors 1, 2, 10, and 11 are suitably used for, for
example, a smoke sensor, a proximity sensor, and a distance
measuring sensor which cannot secure a sufficient volume. Each of
the smoke sensor, the proximity sensor, and the distance measuring
sensor can be realized by a detector using a light-emitting element
and a light-receiving element. The smoke sensor senses a
sensitivity fluctuation depending on a quantity of a smoke which
blocks a space between the light-emitting element and the
light-receiving element. Both the proximity sensor and the distance
measuring sensor causes the light-receiving element to sense a
quantity of light reflected by an object to be detected in response
to light emitted from the light-emitting element. Thus, using the
light receiving sensors 1, 2, 10, and 11 as described earlier for
any of the sensors, the sensor can be beneficially driven with
fewer terminals and with a low voltage.
[0143] Further, as described earlier, the light receiving sensors
1, 2, 10, and 11 allow an operation at a high speed and also allow
securing a sufficient magnitude of an SN ratio of a detection
signal relative to a small photocurrent Ipd. The application of the
light receiving sensors 1, 2, 10, and 11 is beneficial because an
object can be detected precisely at a high speed by applying the
light receiving sensors 1, 2, 10, and 11 to any of the sensors.
[Configuration of Copying Machine]
[0144] Here, the following explains a copying machine as a specific
example of an electronic device that employs an optical sensor.
FIG. 8 is a front view illustrating an internal configuration of
the copying machine 301.
[0145] As shown in FIG. 8, in a copying machine 301, a document
placed on a platen 303 provided in an upper section of a main body
302 is irradiated with light of a light source lamp 304. Then,
light reflected from the document is thrown onto a charged
photosensitive drum 307 via a group of mirrors 305 and a lens 306
and thereby an exposure process is carried out. Further, the
copying machine 301 adheres toner onto a static latent image that
is formed on the photosensitive drum 307 by the exposure process.
As a result, a toner image is formed. Further, the copying machine
301 transfers the toner image on the photosensitive drum 307 onto a
sheet supplied via a sheet carrying system 311 from a manual sheet
feeding tray 308 or a sheet feeding cassette 309 or 310.
Subsequently, the toner image is fixed in a fixing device 312 and
then the sheet is discharged to the outside of the main body
302.
[0146] In the copying machine 301 configured as described above,
for detecting a position of each section or detecting passage of a
sheet, optical sensors S1 to S12 are provided.
[0147] The optical sensors S1 to S4 are provided for detecting a
position of a part of the group of mirrors 305 that move in a light
scanning direction of the document. The optical sensors S5 and S6
are provided for detecting a position of a lens 306 that moves
together with the part of the group of mirrors 305. The optical
sensor S7 is provided for detecting a rotational position of the
photosensitive drum 307.
[0148] The optical sensor S8 is provided for detecting the presence
or absence of a sheet on the manual sheet feeding tray 308. The
optical sensor S9 is provided for detecting the presence or absence
of a sheet that has been fed from the upper sheet feeding cassette
309 and that is being carried. The optical sensor S10 is provided
for detecting the presence or absence of a sheet that has been fed
from the lower sheet feeding cassette 310 and that is being
carried.
[0149] The optical sensor S11 is provided for detecting separation
of a sheet from the photosensitive drum 307. Further, the optical
sensor S12 is provided for detecting discharge of a sheet to the
outside of the copying machine 301.
[0150] As described above, the copying machine 301 includes many
optical sensors S1 to S12. By using light receiving sensors 1, 2,
10, and 11 of any of Embodiments described above as these optical
sensors S1 to S12, a function of the copying machine 301 can be
enhanced by the optical sensors S1 to S12.
[0151] Note that, for convenience of explanation, the optical
sensors S1 to S12 are explained as examples above. However, in an
actual copying machine, a larger number of optical sensors are
often used. Therefore, in such an electronic device, the
above-described effects become more prominent.
CONCLUSION
[0152] Each optical sensor (the light receiving sensors 1, 2, 10,
and 11) according to one aspect of the present invention comprises
the photodiode (the photodiode PD) which generates the photocurrent
(the photocurrent Ipd) upon receipt of light, the first MOS
transistor (the transistors Tr11 and Tr111) through which the
photocurrents flows, the second MOS transistor (the transistors
Tr12 and Tr112) which forms, together with the first MOS
transistor, the current mirror circuit (the first current mirror
circuit CM1), the third MOS transistor (the transistors Tr9 and
Tr10) whose channel type is different from that of the first MOS
transistor, and the current-voltage conversion element (the
resistor R10) which converts, to a voltage, a current flowing
through the second MOS transistor, the current mirror circuit
amplifying the photocurrent, the first MOS transistor having a
source connected with a gate of the third MOS transistor, and the
third MOS transistor having a threshold voltage that is set to be
equal to or above a threshold voltage of the first MOS
transistor.
[0153] Note that the current-voltage conversion element is
typically a resistor, but is not limited to the resistor.
Alternatively, the current-voltage conversion element may be an
element such as a diode.
[0154] The optical sensor is configured such that if the first MOS
transistor is a P-type MOS transistor (the transistor Tr11), both a
drain and a gate of the first MOS transistor are connected with a
cathode of the photodiode. Further, the optical sensor is
configured such that if the first MOS transistor is an N-type MOS
transistor (the transistor Tr111), both a drain and a gate of the
first MOS transistor are connected with an anode of the
photodiode.
[0155] In the above configuration, the photodiode, upon receipt of
light, generates a photocurrent. Accordingly, the photocurrent is
amplified by the first MOS transistor and the second MOS transistor
both of which form the current mirror circuit. Then, the amplified
photocurrent flowing through the second MOS transistor is converted
into a voltage by a resistor, and the voltage is outputted as a
detection signal.
[0156] Further, a threshold voltage of the third MOS transistor is
set to be equal to or above a threshold voltage of the first MOS
transistor. Accordingly, a reverse bias voltage is applied to the
photodiode.
[0157] Accordingly, the photodiode operates in a nearly reverse
bias state. This decreases a capacitance value of the photodiode
and thus increases a response speed of the photodiode. Therefore,
the optical sensor can operate at a high speed.
[0158] Further, the current flowing through the resistor is larger
than the photocurrent as described earlier. Thus, a sufficient
magnitude of the current can be secured even if the photocurrent is
small. Accordingly, it is possible to secure a sufficient magnitude
of an SN of the detection signal with respect to the small
photocurrent.
[0159] It is preferable that the optical sensor is configured such
that a back gate of the first MOS transistor is connected with a
gate of the first MOS transistor.
[0160] In the above configuration, the back gate and the gate of
the first MOS transistor are connected with each other. This allows
a decrease in threshold voltage of the first MOS transistor without
changing a diffusive concentration. This results in decrease of a
leakage current and therefore enables a higher reverse bias voltage
to be applied to the photodiode. Consequently, it is possible to
improve a response speed of the optical sensor.
[0161] The optical sensor comprises an inverter (the transistors
Tr1 and Tr2) which turns on or off the third MOS transistor (the
transistor Tr4) by switching on or off in accordance with a voltage
into which the photocurrent amplified by the current mirror circuit
is converted, the inverter being formed by two inverter MOS
transistors, one of which (the transistor Tr2) has a channel type
that is identical to that of the first MOS transistor and has a
back gate and a gate connected with each other.
[0162] In the above configuration, on-off switching of the third
MOS transistor is controlled by on-off switching of the inverter.
Accordingly, it is possible to increase a speed of an operation of
the third MOS transistor by increasing a speed of an operation of
one of two transistors forming the inverter, especially an inverter
MOS transistor whose channel type is identical to that of the first
MOS transistor.
[0163] A connection between a back gate and a gate of the inverter
MOS transistor decreases a threshold voltage of the inverter MOS
transistor, so that the inverter MOS transistor can be driven with
a low voltage. Accordingly, it is possible to improve a
responsivity of the optical sensor.
[0164] The optical sensor is preferably such that the inverter MOS
transistors are the P-type MOS transistors, and the optical sensor
further includes a diode-connected N-type MOS transistor which is
an auxiliary MOS transistor (the transistor Tr6) connected in
series with the inverter MOS transistor.
[0165] In the above configuration, a decrease in threshold voltage
of the N-type MOS transistor in the optical sensor leads to a
similar decrease in threshold voltage of the auxiliary MOS
transistor. Therefore, it is possible to improve a response
characteristic of the optical sensor.
[0166] An electronic device (the copying machine 301) of one aspect
of the present invention includes any of the above optical sensors.
Accordingly, a use of an optical sensor allows a precise detection
of an object at a high speed.
RIDER
[0167] Further, the present invention is not limited to the
description of the embodiments above, but may be altered within the
scope of the claims. An embodiment based on a combination of
technical means properly altered within the scope of the claims is
encompassed in the technical scope of the present invention.
INDUSTRIAL APPLICABILITY
[0168] The optical sensor of the present invention is configured as
a photo interrupter so as to have a function of detecting, for
example, an object and an operating speed of an object. Thus, the
optical sensor can be applicable suitably to an electric appliance
such as a digital camera, a copying machine, a printer, and a
portable device. Further, the optical sensor of the present
invention is also applicable suitably to a sensor, such as a smoke
sensor, a proximity sensor, and a distance measuring sensor, which
cannot secure a sufficient volume.
REFERENCE SIGNS LIST
[0169] 1 Light receiving sensor (optical sensor) [0170] 2 Light
receiving sensor (optical sensor) [0171] 10 Light receiving sensor
(optical sensor) [0172] 11 Light receiving sensor (optical sensor)
[0173] 101 PchMOS transistor [0174] 102 NchMOS transistor [0175]
113 Gate electrode [0176] 114 Back gate electrode [0177] 115 Back
gate electrode [0178] 301 Copying machine (electronic device)
[0179] CM1 First current mirror circuit (current mirror circuit)
[0180] CM11 First current mirror circuit (current mirror circuit)
[0181] PD Photodiode (photoelectric conversion element) [0182] R10
Resistor (current-voltage conversion element) [0183] Tr2 Transistor
(inverter MOS transistor) [0184] Tr4 Transistor (third MOS
transistor) [0185] Tr6 Transistor (auxiliary MOS transistor) [0186]
Tr9 Transistor (third MOS transistor) [0187] Tr10 Transistor (third
MOS transistor) [0188] Tr111 Transistor (first MOS transistor)
[0189] Tr12 Transistor (second MOS transistor) [0190] Tr111
Transistor (first MOS transistor) [0191] Tr112 Transistor (second
MOS transistor)
* * * * *